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<h1><a name="SECTION0027500000000000000000"></a><a name="profile-limits"></a>
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25.5 Limitations 
</h1>

<p>
One limitation has to do with accuracy of timing information.
There is a fundamental problem with deterministic profilers involving
accuracy.  The most obvious restriction is that the underlying ``clock''
is only ticking at a rate (typically) of about .001 seconds.  Hence no
measurements will be more accurate than the underlying clock.  If
enough measurements are taken, then the ``error'' will tend to average
out. Unfortunately, removing this first error induces a second source
of error.

<p>
The second problem is that it ``takes a while'' from when an event is
dispatched until the profiler's call to get the time actually
<em>gets</em> the state of the clock.  Similarly, there is a certain lag
when exiting the profiler event handler from the time that the clock's
value was obtained (and then squirreled away), until the user's code
is once again executing.  As a result, functions that are called many
times, or call many functions, will typically accumulate this error.
The error that accumulates in this fashion is typically less than the
accuracy of the clock (less than one clock tick), but it
<em>can</em> accumulate and become very significant.

<p>
The problem is more important with <tt class="module">profile</tt> than with the
lower-overhead <tt class="module">cProfile</tt>.  For this reason, <tt class="module">profile</tt>
provides a means of calibrating itself for a given platform so that
this error can be probabilistically (on the average) removed.
After the profiler is calibrated, it will be more accurate (in a least
square sense), but it will sometimes produce negative numbers (when
call counts are exceptionally low, and the gods of probability work
against you :-). )  Do <em>not</em> be alarmed by negative numbers in
the profile.  They should <em>only</em> appear if you have calibrated
your profiler, and the results are actually better than without
calibration.

<p>

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<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
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